Bacteria belonging to the genus Bifidobacterium, which are herein referred to as Bifidobacterium, is one of predominant bacterial strains in intestinal microflora formed in the human intestinal tract. It is known that Bifidobacterium have an intestinal function-regulating activity, an immuno-stimulating activity, and an anti-cancer activity.
Accordingly, demands for food products containing viable Bifidobacterium such as milk fermented with Bifidobacterium or the like are increasing in accordance with an increase of health consciousness of consumers.
Bifidobacterium exhibits a poor proliferation potency in a milk medium. Accordingly, various growth-stimulating substances, such as yeast extract and the like, are generally formulated in fermented milk so that the Bifidobacterium is contained therein at a constant content, for example, 1×107 CFU/ml. However, the growth-stimulating substances are generally expensive and may degrade the taste.
Various methods for promoting the growth of Bifidobacterium or improving the survivability thereof during storage by fermentation with Bifidobacterium and another lactic acid bacterium without adding any growth-stimulating substances or the like have been disclosed. For example, (1) yoghurt containing Lactococcus lactis subsp. lactis, Lactococcus lactis subsp. cremoris, and Bifidobacterium, and a method for preparing the yoghurt have been disclosed (see, for example, Patent Document 1), with respect to a method for promoting the growth of Bifidobacterium to prepare fermented milk.
For example, (2) a method for fermenting milk with Bifidobacterium, including cultivating Bifidobacterium breve together with Lactococcus lactis subsp. lactis, which forms neither diacetyl nor acetoin, in a medium containing milk as the main component thereof has been disclosed (see, for example, Patent Document 2), with respect to a method for improving the survivability of Bifidobacterium during storage of fermented milk.    [Patent Document 1] Japanese Patent Publication No. 3,364,491    [Patent Document 2] Japanese Patent Publication No. 3,068,484